Variable gain amplifier and AC power supply device using the same

ABSTRACT

A variable gain amplifier includes first and second power supply terminals arranged to be connected to a power supply, a transconductance amplifier, first and second PN junction elements, a voltage drop element, first and second resistors, a current-generating transistor, and a current mirror. The transconductance amplifier outputs a current corresponding to a difference between a potential of a base of the first initial stage transistor and a potential of a base of the second initial stage transistor. An emitter of the second initial stage transistor is connected to the emitter of the first initial stage transistor at a node. Each of the first and second PN junction elements has a first end connected to the base of the first initial stage transistor and a second end. The voltage drop element is connected between the second end of the first PN junction element and the first power supply terminal. The first resistor is connected between the base of the second initial stage transistor and a first signal source which is a voltage source. The current-generating transistor has a collector connected to the base of the first initial stage transistor and a base connected to the first signal source. The second resistor is connected between the emitter of the current-generating transistor and the second power supply terminal. The current mirror is connected to the node, and allows a current to flow to the node, the current being identical to a current flowing from the second signal source which is a current source. This variable gain amplifier generates no non-linear distortion, and has a small size.

TECHNICAL FIELD

The present invention relates to a variable gain amplifier and analternating current (AC) power-supply device including the amplifier.

BACKGROUND ART

In a conventional variable gain amplifier disclosed in FIG. 1 ofJapanese Patent Laid-Open Publication No. 2001-308662, thecharacteristic of the base-emitter voltage of each of initial stagetransistors Q₁ and Q₂ versus the collector current is expressed as anexponential function, hence providing an output voltage V0 with anon-linear distortion.

In order to reduce this distortion, a resistor is inserted in series tothe emitter of each of initial stage transistors Q1 and Q2. Thisarrangement may reduce the non-linear distortion, but cannot completelyeliminate the distortion.

A multiplier circuit provides a variable gain amplifier which does notgenerate the non-linear distortion in principle. FIG. 7 is a circuitdiagram of multiplier circuit 5001. Integrated circuit (IC) 1 includestransconductance amplifier 2 and a pair of PN junction elements 4, andincludes seven terminals, that is, power-supply terminal 1A (+Vcc),power-supply terminal 1B (−Vcc), input terminal 1C (INV), input terminal1D (NI), input terminal 1E (BIAS), input terminal 1F (DB), and outputterminal 1G (OUT).

Constant-current supply 101 (ID) is connected to input terminal 1F.Constant power supply 103 (½I_(D)) outputting a ½ of the current fromconstant current supply 101 and signal input power supply (IX) areconnected to input terminal 1C. Current Iout is output from outputterminal 1G.

Amplifier 2 includes four current mirrors 5A to 5D and two initial stagetransistors Q1 and Q2. Pair of PN junction elements 4 are configured bytwo diodes D₁ and D₂.

An operation of multiplier circuit 5001 will be described below.

The following quantity is defined by charge q of an electron, theBoltzmann constant K, and absolute temperature T:VT=K·T/q

Voltage V_(BE) between the base and emitter of a transistor is expressedby the following equation with emitter current I_(E), saturated currentI_(SAT) and collector current I_(C). Base current I_(B) is much smallerthan a collector current, hence providing I_(E)≈I_(C).V _(BE) =V _(T)·ln(I _(E) /I _(SAT))=V _(T)·ln(I _(C) /I _(SAT))

Forward direction voltage VD of a diode is expressed by the followingequation with forward direction current ID of the diode.V _(D) =V _(T)·ln(I _(D) /I _(SAT))

The Kirchhoff's law is applied between points A and B shown in FIG. 7,providing the following equations.V _(T)·ln(I _(D1) /I _(SAT))+V _(T)·ln(I ₁ /I _(SAT))=V _(T)·ln(I _(D2)/I _(SAT))+V _(T)·ln(I ₂ /I _(SAT))  (Equation 1)(I _(D1) /I _(SAT))·(I ₁ /I _(SAT))=(I _(D2) /I _(SAT))·(I ₂ /I _(SAT))I _(D1) ·I ₁ =I _(D2) ·I ₂  (Equation 2)I _(D1)=(½)·I _(D) +I _(X)  (Equation 3)I _(D2)=(½)·I _(D) −I _(X)  (Equation 4)I ₁=(½)·Iy+Iout  (Equation 5)I ₂=(½)·Iy−Iout  (Equation 6)

From Equation 3, Equation 6 is assigned into Equation 2, providing thefollowing equations.Ix·Iy=I _(D) ·IoutIout=Ix·Iy/I _(D)  (Equation 7)

Based upon Equation 7, when current I_(D) is constant, the product ofinput currents Ix and Iy is obtained from output terminal 1G as currentIout. When input current Iy is constant, the dividend obtained bydividing input current Ix by input current I_(D) is obtained as outputcurrent Iout. Equation 7 includes no term of exponential function, andincludes only terms of linear function, hence allowing multipliercircuit 5001 to generate no linear distortion in principle. Inputcurrent Ix and input current Iy are used as a signal input and a controlinput, respectively, thereby providing a variable gain amplifiergenerating no linear distortion in principle.

Multiplier circuit 5001 requires input terminal 1F for inputting acurrent to IC 1 from constant current supply 101, hence causing IC 1 toinclude seven terminals. An IC generally includes an even number ofterminals, thus IC 1 necessarily includes a package having eightterminals.

Under recent demands for light weight and small electronic apparatuses,it is very important whether the IC package includes eight pins or sixpins. In IC 1 accommodating multiplier circuit 5001, an area occupied bypads used for connecting terminals is greater than an area of a chipimplementing circuit 5001. Thus, the area of the pads corresponding tothe number of terminals influences the area of the chip, and is greatlyreflected in the yield and costs.

SUMMARY OF THE INVENTION

A variable gain amplifier includes first and second power supplyterminals arranged to be connected to a power supply, a transconductanceamplifier, first and second PN junction elements, a voltage dropelement, first and second resistors, a current-generating transistor,and a current mirror. The transconductance amplifier outputs a currentcorresponding to a difference between a potential of a base of the firstinitial stage transistor and a potential of a base of the second initialstage transistor. An emitter of the second initial stage transistor isconnected to the emitter of the first initial stage transistor at anode. Each of the first and second PN junction elements has a first endconnected to the base of the first initial stage transistor and a secondend. The voltage drop element is connected between the second end of thefirst PN junction element and the first power supply terminal. The firstresistor is connected between the base of the second initial stagetransistor and a first signal source which is a voltage source. Thecurrent-generating transistor has a collector connected to the base ofthe first initial stage transistor and a base connected to the firstsignal source. The second resistor is connected between the emitter ofthe current-generating transistor and the second power supply terminal.The current mirror is connected to the node, and allows a current toflow to the node, the current being identical to a current flowing fromthe second signal source which is a current source.

This variable gain amplifier generates no non-linear distortion, and hasa small size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a variable gain amplifier in accordancewith an exemplary embodiment of the present invention.

FIG. 2 is a circuit diagram of an integrated circuit of the variablegain amplifier in accordance with the embodiment.

FIG. 3 is a circuit diagram of another integrated circuit of thevariable gain amplifier in accordance with the embodiment.

FIG. 4A is a circuit diagram of another variable gain amplifier inaccordance with the embodiment.

FIG. 4B is a circuit diagram of still another variable gain amplifier inaccordance with the embodiment.

FIG. 5 is a circuit diagram of an alternating-current (AC) power-supplydevice in accordance with the embodiment.

FIG. 6 is a circuit diagram of another AC power-supply device inaccordance with the embodiment.

FIG. 7 is a circuit diagram of a conventional variable gain amplifier.

REFERENCE NUMERALS

-   2 Transconductance Amplifier-   3 Voltage Drop Element-   4A PN Junction Element (First PN Junction Element)-   4B PN Junction Element (Second PN Junction Element)-   5C Current Mirror-   10A Power Supply Terminal-   10B Power Supply Terminal-   10G Output Terminal-   501 Signal Source (First Signal Source)-   502 Signal Source (Second Signal Source)-   FET₁ Electric Field Effect Transistor-   Q₁ Initial Stage Transistor (First Initial Stage Transistor)-   Q₂ Initial Stage Transistor (Second Initial Stage Transistor)-   R₁₀ Resistor (Second Resistor)-   R₁₁ Resistor (First Resistor)-   Tr₁ Current-Generating Transistor

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a circuit diagram of a variable gain amplifier 1001 includinga multiplier circuit in accordance with an exemplary embodiment of thepresent invention. In FIG. 1, components identical to those ofconventional multiplier circuit 5001 shown in FIG. 7 are denoted by thesame reference numerals, and their description will be omitted. Unlikeconventional multiplier circuit 5001 shown in FIG. 7, variable gainamplifier 1001 shown in FIG. 1 does not require input terminal 1F.

As shown in FIG. 1, integrated circuit (IC) 10 includes transconductanceamplifier 2, voltage drop element 3, and a pair of PN junction elements4A and 4B. IC 10 includes six 6 terminals, that is, power supplyterminal 10A (Vcc), input terminal 10C (INV), input terminal 10D (NI),input terminal 10E (BIAS), output terminal 10G (OUT), and power supplyterminal 10B (GND) functioning as a ground. Power supply Vcc has an endVcc1 connected to power supply terminal 10A, and has an end Vcc2connected to power supply terminal 10B.

PN junction elements 4A and 4B are implemented by diodes D₁ and D₂,respectively. An end 3A of voltage drop element 3 is connected to powersupply terminal 10A (Vcc) so that a voltage dropping by a voltage V₃from the voltage of power supply terminal 10A appears at an end 3B ofvoltage drop element 3. Respective anodes of diodes D₁ and D₂ areconnected to the end 3B of voltage drop element 3. The cathode of diodeD₁ is connected to input terminal 10C (INV). The collector ofcurrent-generating transistor Tr₁ is connected to input terminal 10C,and the emitter of current-generating transistor Tr₁ is connected to anend R₁₀A of resistor R₁₀. An end R₁₀B of resistor R₁₀ is connected topower supply Vcc (power supply terminal 10B). Voltage Vs of signalsource 501 is applied to the base of current-generating transistor Tr₁.Signal source 501 is biased by DC bias Vb. Input terminal 10D (NI) isconnected to signal source 501 via resistor R₁₁.

A potential of a node where diode D₁ and the collector ofcurrent-generating transistor Tr₁ is connected and a potential of a nodewhere resistor R₁₁ and diode D₂ are connected are input as differentialinputs to transconductance amplifier 2.

The current flowing in a common emitter of initial stage transistors Q₁and Q₂ providing an initial stage differential amplifier oftransconductance amplifier 2 is controlled via current mirror 5C bycurrent Ic of signal source 502, a current source.

Transconductance amplifier 2 includes initial stage transistors Q1 andQ2. The emitter of initial stage transistor Q1 is connected to theemitter of initial stage transistor Q2 at a node 2P. Current Ioutcorresponding to a difference between a potential of the base of initialstage transistor Q1 and a potential of the base of initial stagetransistor Q2 is output from output terminal 10G. Diode D₁, i.e., PNjunction element 4A, has an end (anode) and an end (cathode) that isconnected to the base of initial stage transistor Q1. Diode D₂, i.e., PNjunction element 4B, has an end (anode) and an end (cathode) that isconnected to the base of initial stage transistor Q2. The cathodes ofdiodes D₁ and D₂ are connected to each other. Voltage drop element 3 isconnected between the cathodes of diodes D₁ and D₂ and power supplyterminal 10A. Resistor R₁₁ is connected between the base of initialstage transistor Q2 and signal source 501, a voltage source. Thecollector of current-generating transistor Tr₁ is connected to the baseof initial stage transistor Q₁. The base of current-generatingtransistor Tr₁ is connected to signal source 501. Resistor R₁₀ isconnected between power supply terminal 10B and the emitter ofcurrent-generating transistor Tr₁. Current mirror 5C is connected to thenode 2P to cause a current identical to current Ic flowing from signalsource 501, the current source, to flow in the node 2P. That is, currentmirror 5C makes the sum of currents flowing through the emitters ofinitial stage transistors Q1 and Q2 equal to current Ic.

In variable gain amplifier 1001 according to the embodiment, while thesum (current I_(D)) of currents I_(D1) and I_(D2) flowing in diodes D₁and D₂ is kept constant, an input signal is added to current I_(D1), andcurrent Ic is applied to the BIAS terminal. Similarly to conventionalmultiplier circuit 5001 shown in FIG. 7, according to Equations 1 to 7,when ID is constant, the product of input currents Is and Ic is obtainedfrom output terminal 10G as current Iout, thus providing a multiplyingfunction.

A method for adding an input signal to current I_(D1) while the sum(current ID) of currents I_(D1) and I_(D2) is kept constant will bedescribed. Each of resistors R₁₀ and R₁₁ has a resistance R. A voltage(0.7V) between the base and emitter of current-generating transistor Tr₁will be neglected.I _(D1)=(Vb+Vs)/RI _(D2)=(Vcc−V ₃−(Vb+Vs))/RI _(D1) +I _(D2)=(Vcc−V ₃)/RThus, the sum (I_(D1)+I_(D2)) of currents I_(D1) and I_(D2) is constantregardless of voltage Vs of signal source 501.

If an offset is to be adjusted, resistor R₁₂ is adjusted so thatcurrents I_(D1) and I_(D2) are equal to each other when no signal isinput (Vs=0). Therefore, upon amplifying an alternating-current (AC)signal without taking a direct-current component into consideration, R₁₂may not be necessary.

In FIG. 1, current-generating transistor Tr₁ and resistors R₁₀ and R₁₁input voltage Vs of signal source 501 to input terminals 10C and 10D asinput current Is. Currents I_(D1) and I_(D2) flowing in diodes D₁ and D₂have phases opposite to each other.

In FIG. 1, a term of Equation 7 is expressed by the following equation.I _(D)=I_(D1) +I _(D2)Ix=IsIy=Ic

From Equation 7, output current Iout is expressed by the followingequation.Iout=Is·Ic/(I _(D1) +I _(D2))

Since (I_(D1)+I_(D2)) is constant regardless of voltage Vs, the gain ofvariable gain amplifier 1001 shown in FIG. 1 is determined by current Icto be input to input terminal 10E. Since this equation includes only theterm of linear function, variable gain amplifier 1001 does not generateany non-linear distortion in principle. That is, the ratio of currentIout output from output terminal 10G to voltage Vs of signal source 501of transconductance amplifier 2 is variable according to current Icflowing from signal source 502.

Diodes D₁ and D₂ and initial stage transistors Q₁ and Q₂ need to be keptat the same temperature so as to perform the multiplying functionaccurately, and are preferably arranged on a single silicon substrate asan IC. Diodes D₁ and D₂, a pair of PN junction elements 4A and 4B,preferably have the same characteristics and the same junctiontemperature, and are preferably placed inside the IC.

As described above, according to the embodiment, IC 10 has sixterminals, and accommodated in a package identical to that of asmall-signal transistor, which is the smallest IC, effectively meetingdemands for thin and small-size electronic apparatuses. IC 10 includesthe six terminals, and has an area of a chip much smaller than that ofan IC including eight pins, being advantageous from the viewpoints ofyield and cost.

FIG. 2 is a circuit diagram of integrated circuit 10 for showingtransconductance amplifier 2 and voltage drop element 3. In the casethat diodes D₁ and D₂ of PN junction elements 4A and 4B are configuredby an IC, transistors with diode-connection, i.e., with the collectorand the base of each of the transistors being connected to each other,are actually used as diodes D₁ and D₂.

Transconductance amplifier 2 is configured by four current mirrors 5A to5D and two initial stage transistors Q₁ and Q₂. Each of current mirrors5A to 5D is configured by three transistors Q₅, Q₆, and Q₇.

Voltage drop element 3 applies appropriate potentials input totransconductance amplifier 2, that is, to the bases of initial stagetransistors Q₁ and Q₂. Element 3 includes two transistors Q₈ and Q₉ thatutilize a forward-direction voltage drop of diode and resistor R₃ as animpedance element connected between transistors Q₈ and Q₉. Voltage dropelement 3 may be an element, such as a zener diode, that drops avoltage.

FIG. 3 is another circuit diagram of integrated circuit 10. As shown inFIG. 3, PN junction elements 4A and 4B are implemented by a PN junctionbetween the base and emitter of each of transistors Q₃ and Q₄.

FIG. 4A is a circuit diagram of variable gain amplifier 1001A accordingto the embodiment. Variable gain amplifier 1001A includes field effecttransistor FET₁ instead of current-generating transistor Tr1 of variablegain amplifier 1001 shown in FIG. 1. The drain, the gate and the sourceof field effect transistor FET₁ are connected instead of the collector,the base, and the emitter of current-generating transistor Tr₁,respectively.

FIG. 4B is a circuit diagram of another IC 110 of variable gainamplifier 1001 according to the embodiment. IC 110 has an arrangement ofvoltage drop element 3, PN junction elements 4A and 4B,current-generating transistor Tr₁, and resistor R₁₀ is opposite to thatof IC 10 shown in FIG. 1, and the other portions are arranged in thesame manner. That is, the anode of diode D₁ is connected to inputterminal 110C and the base of initial stage transistor Q₁, and the anodeof diode D₂ is connected to input terminal 110D and the base oftransistor Q₂. The cathodes of diode D1 and D2 are connected to end 3Aof voltage drop element 3. End 3B of voltage drop element 3 is connectedto power supply terminal 110B connected to the ground. IC 110 providesthe same effects as those of IC 10 shown in FIG. 1. In FIG. 4B, resistorR₁₂ for adjusting offset is omitted.

FIG. 5 is a circuit diagram of showing alternating-current (AC) constantvoltage power supply device 2001 including variable gain amplifier 1001according to the embodiment. Wave-form generator 11 generates a waveformof an AC voltage to be desirably obtained as an output. In order togenerate an AC voltage having a sine wave form, wave-form generator 11generates a sine wave voltage. The sine wave voltage output fromwave-form generator 11 is applied to current-generating transistor Tr₁and resistor R₁₁ so that sine wave current Iout is obtained from outputterminal 10G of IC 10. Resistor R₁₇ converts sine wave current Ioutoutput from IC 10 into a sine wave voltage. This sine wave voltage isinput to power amplifier 12.

The sine wave voltage input to power amplifier 12 is amplified, and isapplied to primary coil N₁ of transformer 13 for driving transformer 13.A sine wave AC voltage is output from secondary coil N₂ of transformer13 via output terminals 13A and 13B.

Tertiary coil N₃ of the transformer generates an AC voltage inproportion to the output AC voltage, and the AC voltage is convertedinto a pulsating voltage by a rectifier circuit configured by diodesD₁₀, D₁₁, D₁₂, and D₁₃ and resistor R₁₉. Resistor R₁₉ is a dummy loadresistor for obtaining an appropriate pulsating voltage. The pulsatingvoltage is smoothed by a low pass filter configured by resistor R₁₄ andcapacitor C₁₀, providing a direct-current (DC) voltage in proportion tothe AC voltage output from output terminals 13A and 13B. That is, therectifier circuit and the low pass filter provides output voltagedetector 14 that detects the AC voltage output from output terminals 13Aand 13B, and generates the DC voltage as a detection signal.

Comparator OP₁ compares the DC voltage with reference voltage Vref, andoutputs a voltage corresponding to a difference obtained by subtractingthe DC voltage from reference voltage Vref. The output voltage is inputto input terminal 10E of IC 10 as current Ic via resistor R₁₃. If the DCvoltage is lowered to increase the difference from reference voltageVref, the voltage output from by comparator OP1 increases as to allowvariable gain amplifier 1001 to increase the AC voltage output fromoutput terminals 13A and 13B. The AC voltage output through such anegative feedback is stabilized. If the output voltage is changed,reference voltage Vref is changed, or the voltage output from outputvoltage detector 14 is voltage-divided and changed. Resistors R₁₅ andR₁₆ and capacitor C₁₁ constitute a phase compensating circuit used forstably performing the negative feedback. Output voltage detector 14,comparator OP₁, reference voltage Vref, resistors R₁₃, R₁₅, and R₁₆, andcapacitor C₁₁ provide a feedback circuit that feedbacks the amplitude ofthe output AC voltage as current Ic.

FIG. 6 is a circuit diagram of AC constant current power-supply device3001 in accordance with this embodiment. In FIG. 6, components identicalto those of AC constant voltage power supply device 2001 are denoted bythe same reference numerals, and their description will be omitted.Resistor R₁₈ generates an AC voltage in proportion to an AC currentflowing in output terminals 13A and 13B. The generated AC voltage isconverted into a DC voltage by a rectifier circuit including diodes D₁₅and D₁₄ and a low pass filter including capacitor C₁₀ and resistor R₁₄.That is, the rectifier circuit and the low pass filter provides outputcurrent detector 15 for detecting an AC current flowing in outputterminals 13A and 13B, and outputs a DC voltage in proportional to thecurrent. Similarly to constant voltage power supply device 2001 shown inFIG. 5, comparator OP₁ outputs a voltage corresponding to a differenceobtained by subtracting the DC voltage from reference voltage Vref. IC10 drives transformer 13 via power amplifier 12 in accordance with thevoltage output from comparator OP₁ so that a DC voltage output fromoutput current detector 15 becomes identical to reference voltage Vref.This operation stabilizes the current flowing through output terminals13A and 13B as a constant current. Resistor R₁₉ is a dummy load resistorfor setting a pulsating voltage output from the rectifier circuit to anappropriate level. Output current detector 15, comparator OP₁, referencevoltage Vref, and resistor R₁₁ provide a feedback circuit that feedbacksthe amplitude of the output AC voltage as current Ic.

Variable gain amplifier 1001 in accordance with the embodiment providesAC power supply devices 2001 and 3001 outputting a constant voltage anda constant current which are variable, respectively.

INDUSTRIAL APPLICABILITY

This variable gain amplifier generates no non-linear distortion, and hasa small size, being applicable to an AC power supply device outputting avariable current. This, variable gain amplifier is useful for variouspower supply devices and power systems, such as uninterruptive powersupply devices and electrophotographic processing power supply devicesfor photocopy machines, printers.

1. A variable gain amplifier comprising: a first power supply terminalarranged to be connected to a first end of a power supply; a secondpower supply terminal arranged to be connected to a second end of thepower supply; a transconductance amplifier including a first initialstage transistor having an emitter, a base and a collector, a secondinitial stage transistor having an emitter connected to the emitter ofthe first initial stage transistor at a node, a base and a collector,and an output terminal for outputting a current corresponding to adifference between a potential of the base of the first initial stagetransistor and a potential of the base of the second initial stagetransistor; a first PN junction element having a first end connected tothe base of the first initial stage transistor and a second end; asecond PN junction element having a first end connected to the base ofthe second initial stage transistor and a second end connected to thesecond end of the first PN junction element; a voltage drop elementconnected between the second end of the first PN junction element andthe first power supply terminal; a first resistor connected between thebase of the second initial stage transistor and a first signal sourceserving which is a voltage source; a current-generating transistorhaving a collector connected to the base of the first initial stagetransistor, a base connected to the first signal source, and an emitter;a second resistor connected between the emitter of thecurrent-generating transistor and the second power supply terminal; anda current mirror connected to the node, the current mirror allowing acurrent to flow to the node, the current being identical to a currentflowing from a second signal source which is a current source.
 2. Thevariable gain amplifier according to claim 1, wherein a ratio of thecurrent output from the output terminal to a voltage of the first signalsource of the transconductance amplifier is variable according to thecurrent flowing from the second signal source.
 3. The variable gainamplifier according to claim 1, further comprising an integrated circuitaccommodating the first PN junction element, the second PN junctionelement, the transconductance amplifier, and the current mirror, theintegrated circuit including a first input terminal connected to thebase of the first initial stage transistor, a second input terminalconnected to the base of the second initial stage transistor, a thirdinput terminal which is a node where the current mirror is connected tothe second signal source, the first power supply terminal, the secondpower supply terminal, and the output terminal of the transconductanceamplifier.
 4. The variable gain amplifier according to claim 1, furthercomprising a field effect transistor instead of the current-generatingtransistor, wherein a drain, a gate, and a source of the field effecttransistor are connected instead of the collector, the base, and theemitter of the current-generating transistor, respectively.
 5. Analternating-current (AC) power supply device comprising: the variablegain amplifier according to any one of claims 1 to 3; a wave-formgenerator serving as the first signal source; a power amplifieramplifying the current output from the output terminal of thetransconductance amplifier, and outputs an AC power; and a feedbackcircuit that feedbacks an amplitude of the output AC power as thecurrent flowing from the second signal source.